Office chairs have been developed where seat and back assemblies thereof are tiltable forwardly and rearwardly. Further, one type of office chair is commonly referred to as a "synchro-tilt" type chair wherein the back assembly tilts synchronously with respect to the seat assembly but at a greater rate. As a result, the back assembly tilts relative to the seat assembly as the latter tilts relative to a chair base on which the seat and back are supported. While numerous improvements to these chairs have been made to improve the comfort of a user, for example, with respect to the synchronous tilt mechanism or with respect to the design of the seat and back assemblies, the tilting of the seat and back assemblies in office chairs primarily permits tilting forwardly and rearwardly in a single vertical plane.
For example, a number of office chairs use plastic inner shells for the seat and back which are formed separately in two pieces or together as a single L-shaped piece. The inner shell for the chair back, however, is supported by a rigid back upright member that is pivotally connected to the seat assembly so as to move forwardly and rearwardly. While the plastic inner shell has some flexibility, such chairs typically use rigid armrests which limit the range of motion of a user. Also, the upright member is rigid which limits the flexure of the chair shell particularly in the lumbar region of a user. Further, these chairs typically include plastic outer shells which are secured to and cover the back surface of the inner shell and effectively limit flexing of the inner shell even further.
Previous attempts have been made to provide chairs which have an expanded range of motion primarily in the backrest.
In one example, U.S. Pat. No. 148,380 (Perrenet) discloses a chairback joined to the seat by a ball-and-socket joint which permits movement in most directions except in a backward direction. This arrangement also includes a harness which is secured onto the shoulders of a user.
In another example, U.S. Pat. No. 3,552,797 (D'Houdain) discloses a chair for dental treatment having a stationary seat and a backrest. The backrest has an upper section which pivots about an axis Y-Y' that is located at a height spaced upwardly above the waist of a user seated thereon. The lower section of the backrest, however, supports a tilt mechanism for rearward tilting of the seat back about an axis X--X which is located above the seat of a user.
Accordingly, it is an object of this invention to provide an improved office-type chair which provides for lateral tilting of the back assembly relative to a seat assembly and particularly, has a laterally movable back which is self-centering so as to normally maintain the chairback in a vertical upright position. It is a further object that the back assembly be tiltable laterally or sidewardly about a first generally horizontal pivot axis which extends in a forward-rearward direction while the seat assembly is tiltable forwardly and rearwardly about a second horizontal pivot axis which extends sidewardly. It is an object therefore that this improved chair provide three-dimensional tilting where the seat assembly is movable forwardly and rearwardly and the back assembly is movable laterally. It is still a further object that the first pivot axis of the back be located below the level of the seat assembly such that the entire back is movable sidewardly and that the amount of force required for lateral movement of the back be adjustable.
In view of the foregoing, the invention relates to an office-type chair which includes a seat assembly and back assembly that are pivotally supported on a chair base or pedestal to support a user thereon. To increase the comfort of the user, the seat assembly is tiltable forwardly and rearwardly by way of a tilt control mechanism while the back assembly thereof is tiltable laterally from side to side, i.e. in the leftward and rightward directions by way of a back torsion mechanism.
Generally with respect to the main components of the chair, the base is adapted to be supported on a floor and the seat assembly is mounted to the base by the tilt control mechanism. The tilt control mechanism thereby permits forward and rearward tilting of the seat assembly relative to the base, which forward and rearward tilting is conventional. Further, the inventive chair includes the back torsion mechanism which joins the back assembly to the seat assembly. The back torsion mechanism thereby provides a fixed connection therebetween such that the back assembly pivots rearwardly in combination with rearward tilting of the seat assembly. At the same time, the back torsion mechanism also defines a forwardly extending horizontal pivot axis whereby the back assembly can be pivoted to the left and right sides. This combination of forward-rearward tilting and torsional movement thereby accommodates the movements of a user.
The back torsion mechanism not only permits lateral tilting of the back assembly, but also is self-centering in that it includes self-centering means for returning the back assembly to a normally upright position.
More particularly, the back torsion mechanism generally includes a pair of mounting plates which are welded to the back support member so as to move therewith and support a hollow cylindrical bearing therein. The upright member of the back assembly includes a shaft projecting forwardly therefrom which is slidably received within the cylindrical bearing so as to define the horizontal pivot axis extending forwardly and rearwardly about which the back assembly is sidewardly movable. The first horizontal pivot axis preferably is disposed below the level of the seat assembly such that the entire back is movable sidewardly.
The back torsion mechanism also includes self-centering means which normally maintains the back assembly in the vertical central position while permitting the reversible sideward movement thereof. The centering means preferably includes a transverse pin which projects radially outwardly from a front end of the rotatable shaft. The transverse pin seats within corresponding camming grooves on a front surface of the bearing. The camming grooves are defined by an arcuate bearing surface along which the transverse pin can slide during rotation of the back assembly. A compression spring tends to urge the transverse pin back into the camming groove so as to seat in the deepest portion thereof whereby the back assembly is returned to the vertical upright position.
Preferably, the back torsion mechanism also includes tension adjustment means for adjusting the force being applied by the centering spring which increases and decreases the resistance to torsional movement. Further, the back torsion mechanism also includes a locking arrangement to selectively lock out the torsional movement if desired.
While the mounting plates and bearing are formed as separate components, these components may also be formed in a further embodiment as a single metal part which is welded onto the back support member. This metal component is formed with camming grooves and thereby operates substantially the same as the embodiment described above.
Still further, while the first and second embodiments are usable in chairs in both one-piece and two-piece flexible shells, another embodiment of the back torsion mechanism may also be provided in an office chair having the one-piece inner shell instead of separate seat and back assemblies. In this arrangement, the chair includes an L-shaped upright member which is pivotally connected at a front end thereof to a tilt control mechanism. The upright member extends both rearwardly and upwardly to provide support to the back of the one-piece inner plastic shell.
The L-shaped upright member preferably is formed of a horizontal member pivotally connected to the tilt control mechanism, and a vertical member. The horizontal and vertical members are joined together by pivot means such as a thrust bearing such that the vertical member pivots sidewardly relative to the horizontal member.
When the one-piece inner plastic shell is connected both to the tilt control mechanism and the vertical portion of the upright member, the inner plastic shell serves as centering means for the tilt control mechanism. In particular, the inner shell is resiliently flexible such that a back portion thereof is movable sidewardly relative to a seat portion thereof. The seat portion, however, is fixed in place on the tilt control mechanism. Since the inner shell is resiliently flexible, the shell urges the seat back to a normally upright position when not in use.